1. Field of the Invention
The present invention relates to a screen for an optical instrument, e.g., a microfilm reader, which can be under used normal room illumination conditions (hereinafter referred to as a rear projection screen). More particularly, the present invention is concerned with a rear projection screen having a luminance range sufficient to faithfully reproduce the density range of a projected image.
2. Description of the Prior Art
Recently, high performance rear projection screens have been required with the development of microfilm technology. In particular, as an image to be projected changes from a character or line drawing to one of a wide density region and continuous gradation, e.g., an X-ray photograph for medical use and a color image, much higher performance has been required.
The properties required for such a rear projection screen are as follows:
1. Desired light redistribution characteristic
Light is uniformly redistributed only in the predefined audience viewing range in such a manner that the luminance viewed from any point in the audience viewing range is substantially constant.
2. Low scintillation
This property is very important with respect to the ease of viewing and viewing fatigue.
3. Wide image-reproduction range (high contrast)
To enlarge the image reproduction range of a screen, the value of the equation: ##EQU1## (I.E., THE SIGNAL/NOISE RATIO) MUST BE MADE AS LARGE AS POSSIBLE. For this purpose, one can:
A. Increase the luminance of the light source of the projection unit.
B. Increase the transmittance and decrease the reflectance of a screen diffusion plate.
C. Use a transparent plastic sheet having the louver structure in the form of Venetian blind. 4. High resolving power
An improved rear projection screen based on the above considerations, in which a means for shielding room light in the form of Venetian blind is used, is shown in FIG. 1. This screen is described in U.S. Pat. No. 2,053,173, in which transparent plastic sheets with a black varnish coated on the surfaces thereof are laminated and cut at a right angle to the surface of the laminated sheet and the resulting sheet is matted on one side and used as an image-forming surface of a screen.
In this screen, a number of light-shielding layers in the form of Venetian blind, which are at right angles to the surface of the sheet (hereinafter referred to as a louver), shield room light incident upon the screen at an angle exceeding a certain angle, and thus they act to broaden the reproduction range (contrast) of a screen.
Plastic sheets of such a louver structure are used not only as a projection screen, but also for the purposes of:
1. increasing contrast by decreasing the influence of room light.
2. reducing scintillation, and
3. limiting the viewing angle.
For example, they can be mounted on the surface of instrument panels for the purpose of preventing surface reflection, or they can be mounted on a CRT display surface to improve the contrast thereof, or they can be used as directional projectors in combination with a light source.
The above properties which result from the louver structure are preferred in a rear projection screen.
The rear projection screen shown in FIG. 1 has the advantage that the image reproduction range is markedly broadened because the louver structure prevents reflection of room light. This screen, however, has the serious disadvantages explained hereinafter.
Firstly, in the case of a rear projection screen of a structure as shown in FIG. 1, light coming from the matted surface appears as scintillation, which is very unpleasant to an observer and causes eye fatigue. Since the matted surface fails to sufficiently transmit light to form an image due to high reflectance thereof and the reflected light is visible to an observer as stray light, the reproduction range (contrast) of the screen is reduced, and thus the effect of the louver structure is reduced.
It has been proposed to coat light-diffusing particles dispersed in a binder in place of the matted surface on one side of a transparent plastic sheet of the louver structure. In this case, however, scintillation is high and the reflectance of the light diffusion layer is high, and thus this modification suffers from the same defects as with the use of a matted surface.